Pulse detection method and apparatus using patient impedance

ABSTRACT

The presence of a cardiac pulse in a patient is determined by evaluating fluctuations in an electrical signal that represents a measurement of the patient&#39;s transthoracic impedance. Impedance signal data obtained from the patient is analyzed for a feature indicative of the presence of a cardiac pulse. Whether a cardiac pulse is present in the patient is determined based on the feature in the impedance signal data. Electrocardiogram (ECG) data may also be obtained in time coordination with the impedance signal data. Various applications for the pulse detection of the invention include detection of PEA and prompting PEA-specific therapy, prompting defibrillation therapy and/or CPR, and prompting rescue breathing depending on detection of respiration.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/013,941, filed on Dec. 6, 2001, which is hereby incorporated byreference herein.

FIELD OF THE INVENTION

The invention relates generally to the detection of cardiac activity ina patient, and more specifically, to a method and apparatus for cardiacpulse detection.

BACKGROUND OF THE INVENTION

The presence of cardiac pulse in a patient is presently detectedpreferably by palpating the patient's neck and sensing changes in thevolume of the patient's carotid artery due to blood pumped from thepatient's heart. If a pulse can be felt at the carotid artery, it islikely that the patient's heart is pumping sufficient blood to supportlife. A graph representative of the physical expansion and contractionof a patient's carotid artery during two consecutive pulses, orheartbeats, is shown at the top of FIG. 1. When the heart's ventriclescontract during a heartbeat, a pressure wave is sent throughout thepatient's peripheral circulation system. The carotid pulse shown in FIG.1 rises with the ventricular ejection of blood at systole and peaks whenthe pressure wave from the heart reaches a maximum. The carotid pulsefalls off again as the pressure subsides toward the end of each pulse.

An electrocardiogram (ECG) waveform describes the electrical activity ofa patient's heart. The middle graph of FIG. 1 illustrates an example ofan ECG waveform for two heartbeats corresponding in time with thecarotid pulse. Referring to the first shown heartbeat, the portion ofthe ECG waveform representing depolarization of the atrial muscle fibersis referred to as the “P” wave. Depolarization of the ventricular musclefibers is collectively represented by the “Q,” “R,” and “S” waves of theECG waveform. Finally, the portion of the waveform representingrepolarization of the ventricular muscle fibers is known as the “T”wave. Between heartbeats, the ECG waveform returns to an isopotentiallevel.

Discussed herein with respect to the present invention is thecorrelation of fluctuations in a patient's transthoracic impedance withblood flow that occurs with each cardiac pulse wave. The bottom graph ofFIG. 1 illustrates an example of a filtered impedance signal for apatient in which fluctuations in impedance correspond in time with thecarotid pulse and ECG waveform.

The lack of a detectable cardiac pulse in a patient is a strongindicator of cardiac arrest. Cardiac arrest is a life-threateningmedical condition in which the patient's heart fails to provide enoughblood flow to support life. During cardiac arrest, the electricalactivity may be disorganized (ventricular fibrillation), too rapid(ventricular tachycardia), absent (asystole), or organized at a normalor slow heart rate (pulseless electrical activity). A caregiver mayapply a defibrillation shock to a patient in ventricular fibrillation(VF) or ventricular tachycardia (VT) to stop the unsynchronized or rapidelectrical activity and allow a perfusing rhythm to commence. Externaldefibrillation, in particular, is provided by applying a strong electricpulse to the patient's heart through electrodes placed on the surface ofthe patient's body. If a patient lacks a detectable pulse but has an ECGrhythm of asystole or pulseless electrical activity (PEA), anappropriate therapy includes cardiopulmonary resuscitation (CPR), whichcauses some blood flow.

Before providing defibrillation therapy or CPR to a patient, a caregivermust first confirm that the patient is in cardiac arrest. In general,external defibrillation is suitable only for patients that areunconscious, apneic (i.e., not breathing), pulseless, and in VF or VT.Medical guidelines indicate that the presence or absence of a pulse in apatient should be determined within 10 seconds. See, “American HeartGuidelines 2000 for Cardiopulmonary Resuscitation and EmergencyCardiovascular Care, Part 3: Adult Basic Life Support,” Circulation 102suppl. I: I-22-I-59, 2000.

Unfortunately, under the pressures of an emergency situation, it can beextremely difficult for first-responding caregivers with little or nomedical training to consistently and accurately detect a cardiac pulsein a patient (e.g., by palpating the carotid artery) in a short amountof time such as 10 seconds. See, Eberle B., et al., “Checking theCarotid Pulse Diagnostic Accuracy of First Responders in Patients Withand Without a Pulse” Resuscitation 33: 107-116, 1996. Nevertheless,because time is of the essence in treating cardiac arrest, a caregivermay rush the preliminary evaluation, incorrectly conclude that thepatient has no pulse, and proceed to provide defibrillation therapy whenin fact the patient has a pulse. Alternatively, a caregiver mayincorrectly conclude that the patient has a pulse and erroneouslywithhold defibrillation therapy. A need therefore exists for a methodand apparatus that quickly, accurately, and automatically determines thepresence of a pulse in a patient, particularly to prompt a caregiver toprovide defibrillation or CPR therapy, as appropriate, in an emergencysituation.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus that determinesthe presence of a cardiac pulse in a patient by evaluating fluctuationsin an electrical signal that represents a measurement of the patient'stransthoracic impedance. By physiologically associating impedancefluctuations with the presence of a cardiac pulse, the presence orabsence of a cardiac pulse in the patient is determined.

In accordance with one aspect of the present invention, impedance signaldata obtained from a patient is analyzed for a feature indicative of thepresence of a cardiac pulse. Whether a cardiac pulse is present in thepatient is determined based on the feature in the impedance signal data.

The feature in the impedance signal data may be obtained from evaluatingan amplitude of the impedance signal data, an energy in the impedancesignal data, and/or a pattern match statistic resulting from comparingthe impedance signal data with a previously-identified impedance signalpattern known to predict the presence of a cardiac pulse. Otherdetermination and classification techniques known in the art may be usedfor the evaluation.

As to evaluating the amplitude of the impedance signal data, low andhigh peak amplitude values in the impedance signal data may be locatedand the peak-to-peak change in the amplitude from the low to the highpeak amplitude value may be calculated. The peak-to-peak change inamplitude, constituting a feature indicative of the presence of acardiac pulse, may be compared to a threshold to determine the presenceof a pulse.

As to evaluating energy in the impedance signal data, an energycalculation may be performed using impedance signal data obtained fromthe patient. The calculated energy, constituting feature indicative ofthe presence of a cardiac pulse, is compared to a predeterminedthreshold to determine whether a cardiac pulse is present in thepatient.

As to analyzing the impedance signal data using pattern matching, theimpedance signal data may be compared to a previously-identifiedimpedance signal pattern known to predict the presence of a cardiacpulse. The comparison produces a pattern match statistic, constitutingthe feature indicative of the presence of a cardiac pulse, which iscompared to a predetermined threshold to determine whether a cardiacpulse is present in the patient.

The impedance signal data may be obtained from the patient usingdefibrillation electrodes placed on the patient or using separateimpedance-sensing electrodes placed on the patient. If separateelectrodes are used, the present invention includes promptingapplication of the defibrillation electrodes to the patient if a cardiacpulse is determined not present in the patient.

In accordance with another aspect of the present invention,electrocardiogram (ECG) data is obtained from the patient in timecoordination with the impedance signal data. A QRS complex located inthe ECG data is used to select a segment of the impedance signal datafor further analysis. If a pulse is present in the patient, the pulseshould be detectable following the located QRS complexes.

In accordance with yet another aspect of the present invention,pulseless electrical activity (PEA) may be detected when the patient isdetermined pulseless and the patient is not experiencing ventriculardefibrillation (VF), ventricular tachycardia (VT), or asystole. Incircumstances where PEA is found present, the present invention includesprompting delivery of PEA-specific therapy to the patient. The presentinvention may be employed in a variety of devices that providemonitoring and/or therapy. If, for example, the patient is determinedpulseless and experiencing VT with a pulse rate greater than 100 beatsper minute, the present invention may prompt delivery of adefibrillation pulse. If a cardiac pulse is later found in the patientafter delivery of the defibrillation pulse, the present invention mayreport the return of spontaneous circulation in the patient.

The present invention is further useful in evaluating capture whiledelivering pacing stimuli to a patient. If a cardiac pulse is notdetected immediately following a pacing pulse, the current level of thepacing pulse may be increased until capture by the pacing stimuli isachieved.

Other applications and advantages of the present invention are readilyapparent. For example, the invention may be implemented in an automatedexternal defibrillator (AED) that prompts the user to performcardiopulmonary resuscitation (CPR) based on the absence of a pulse in apatient. The AED may also prompt the user to provide rescue breathingdepending on detection of respiration. In regard to the latter, theimpedance signal data and other relevant information, such as thepatient's ECG, may be analyzed to detect the presence of respiration inthe patient.

Embodiments of the invention intended for trained medical personnel mayprovide a display of the impedance signal data that is representative ofthe presence or absence of a pulse in a patient. In that regard, theimpedance signal data may be shown as a way form, as shown in FIG. 1.The impedance signal data may also be displayed as a bar.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a pictorial diagram of a carotid pulse waveform, anelectrocardiogram (ECG) waveform, and a filtered transthoracic impedancesignal for two consecutive heartbeats reflecting fluctuations intransthoracic impedance that correspond with pulsatile blood flow;

FIG. 2 is a pictorial diagram of a defibrillator and electrodesconstructed in accordance with the present invention and attached to apatient;

FIG. 3 is a block diagram of major components of the defibrillator shownin FIG. 2;

FIG. 4 is a flow diagram of a pulse detection process performed inaccordance with the present invention;

FIG. 5 is a flow diagram of a pulse rate analysis performed inaccordance with the present invention the pulse detection process shownin FIG. 4;

FIG. 6 is a flow diagram of another pulse detection process performed inaccordance with the present invention in which an impedance signalpattern analysis is performed without an ECG signal analysis;

FIG. 7 is a flow diagram of a protocol implemented by a defibrillator asshown in FIG. 2 that incorporates a pulse detection process provided bythe present invention;

FIG. 8 is a flow diagram of protocol implemented by the defibrillatorshown in FIG. 2 that incorporates a pulse detection process provided bythe present invention;

FIG. 9 is a flow diagram of still another protocol implemented by thedefibrillator shown in FIG. 2 that incorporates a pulse detectionprocess provided by the present invention;

FIG. 10 is a flow diagram of an auto-capture detection process forcardiac pacing that uses a pulse detection process of the presentinvention; and

FIG. 11 is a flow diagram of a patient condition advisory process foruse in a manual defibrillator or monitor which incorporates a pulsedetection process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A device constructed in accordance with the present invention usesmeasurements of a patient's transthoracic impedance to determine thepresence of a cardiac pulse in the patient. As will be appreciated fromthe description herein, the device may be a stand alone unit or it maybe incorporated into another monitoring or therapy-providing device. Inone suitable application, the present invention is implemented in adefibrillator, such as the defibrillator 10 shown in FIG. 2. A patient40 is connected to the defibrillator 10 via electrodes 12, 14 placed onthe skin of the patient. The defibrillator 10 uses the electrodes 12, 14to deliver defibrillation pulses to the patient 40. The defibrillator 10also uses the electrodes 12, 14 to obtain ECG signals from the patient40.

The electrodes 12, 14 are further configured to communicate animpedance-sensing signal through the patient 40. The impedance-sensingsignal is used by the defibrillator 10 to observe the patient'simpedance. Alternatively, the defibrillator 10 may use sensors 20, 22that are separate from the electrodes 12, 14 for communicating theimpedance-sensing signal through the patient. The sensors 20, 22 may beconnected to the electrodes 12, 14, as shown in FIG. 2, they may beattached to the patient 40 via separate wires (not shown) connected tothe defibrillator 10. In either case, the sensors 20, 22 may be suitablyconstructed from standard external patient electrodes known in the art.

The defibrillator 10 measures the impedance of a patient between theelectrodes 12, 14 (or between the sensors 20, 22, as the case may be)when placed on the patient 40. An impedance measuring component of thedefibrillator 10 is preferably used to measure the patient's impedance.

A preferred embodiment of the invention uses a high-frequency, low-levelconstant current technique to measure the patient's transthoracicimpedance, though other known impedance measuring techniques may beused. A signal generator included in the defibrillator 10 produces alow-amplitude, constant current, high-frequency signal (typicallysinusoidal or square). The signal is preferably generated having afrequency in the range of 10 kHz-100 kHz. The current flows between theelectrodes 12 and 14. The resulting current flow causes a voltage todevelop across the patient's body that is proportional to the product ofthe patient's impedance and the applied current. To calculate thepatient's impedance, the impedance measuring component in thedefibrillator 10 divides the measured sensing voltage by the appliedcurrent. Since the measured voltage is linearly related to the patient'simpedance, the impedance signal data used herein may either be acalculated impedance signal or the measured voltage signal.

While embodiments of the invention specifically described herein areshown implemented in a defibrillator 10, the present invention is notlimited to such specific type of application. Those of ordinary skill inthe art will recognize that the advantages of the invention maysimilarly be achieved by implementing the present invention in cardiacmonitors and other types of medical equipment that do not necessarilyprovide defibrillation therapy.

Prior to discussing various pulse detection processes that thedefibrillator 10 may implement in accordance with the present invention,a brief description of certain major components of the defibrillator 10is provided. Referring to FIG. 3, the defibrillator 10 includesdefibrillation electrodes 30 (e.g., electrodes 12, 14 described above inFIG. 2). An impedance-sensing signal generator 56 communicates animpedance-sensing signal through the patient via the electrodes 30. Asignal amplifier 32 amplifies the impedance-sensing signal to a levelappropriate for digitization by analog-to-digital (A/D) converter 36. Abandpass filter 34 filters the amplified impedance-sensing signal toisolate the portion of the signal that most closely reveals fluctuationsdue to blood flow from cardiac pulses. In one embodiment of theinvention, the bandpass filter 34 is a 1-10 Hz bandpass filter.Fluctuations in the impedance signal below 1 Hz are more likely to becaused by respiration in the patient, and not blood flow. Accordingly,the bandpass filter attenuates that component of the impedance signal.The portion of the impedance signal exceeding 10 Hz is more likelyaffected by surrounding noise and is likewise filtered out.

The filtered impedance signal is delivered to the A/D converter 36 thatconverts the impedance signal into digital impedance data for furtherevaluation. The bandpass filter 34 or other filter may be provided toreduce any aliasing introduced in the impedance signal by the A/Dconverter 36. The parameters of such filtering depend, in part, on thesampling rate of the A/D converter. Bandpass and antialiasing filters,as well as A/D converters, are well-known in the art, and may beimplemented in hardware or software, or a combination of both. Forexample, a preferred embodiment uses a hardware lowpass filter on theimpedance signal before the A/D converter 36, and then a softwarehighpass filter on the digital impedance data after the A/D conversion.Another preferred embodiment additionally uses a software lowpass filterafter the A/D conversion to further limit the bandwidth of the impedancesignal. The A/D converter 36 delivers the digital impedance signal datato the processing unit 38 for evaluation.

The processing unit 38 evaluates the impedance signal data for thepresence of a cardiac pulse. The processing unit 38 is preferablycomprised of a computer processor that operates in accordance withprogrammed instructions stored in a memory 40 that implement a pulsedetection process 42, described in more detail below. The processingunit 38 may also store in the memory 40 the impedance signal dataobtained from the patient, along with other event data and ECG signaldata. The memory 40 may be comprised of any type or combination of typesof storage medium, including, for example, a volatile memory such as adynamic random access memory (DRAM), a nonvolatile static memory, orstorage media such as a magnetic tape or disk drive or optical storageunit (e.g., CD-RW).

The processing unit 38 may report the results of the pulse detectionprocess to the operator of the defibrillator 10 via a display 48. Theprocessing unit 38 may also prompt actions (e.g., CPR) to the operatorto direct the resuscitation effort. The display 48 may include, forexample, lights, audible signals, alarm, printer, or display screen. Theprocessing unit 38 may also receive input from the operator of thedefibrillator 10 via an input device 46. The input device 46 may includeone or more keys, switches, buttons, or other types of user inputdevices.

The defibrillation electrodes 30 may further be used to sense thepatient's electrocardiogram (ECG) signals. ECG signals obtained from thepatient may be amplified and filtered in a conventional manner, andconverted into digitized ECG data for evaluation by the processing unit38.

Preferably, the processing unit 38 evaluates the ECG signals inaccordance with programmed instructions stored in the memory 40 thatcarry out an ECG evaluation process 44 to determine whether adefibrillation shock should be provided. A suitable method fordetermining whether to apply a defibrillation shock is described in U.S.Pat. No. 4,610,254, which is assigned to the assignee of the presentinvention and incorporated by reference herein. If the processing unit38 determines that delivery of a defibrillation pulse is appropriate,the processing unit 38 instructs a defibrillation pulse generator 50 toprepare to deliver a defibrillation pulse to the patient. In thatregard, the defibrillation pulse generator 50 uses an energy source(e.g., battery) to charge one or more defibrillation capacitors in thedefibrillator 10.

When the defibrillation charge is ready for delivery, the processingunit 38 advises the operator via the display 48 that the defibrillator10 is ready to deliver the defibrillation pulse. The processing unit 38may ask the operator to initiate the delivery of the defibrillationpulse. When the operator initiates delivery of the defibrillation pulse(e.g., via the input device 46), the processing unit 38 instructs thedefibrillation pulse generator 50 to discharge through the patient theenergy stored in the defibrillation capacitors (via the electrodes 30).Alternatively, the processing unit 38 may cause the defibrillation pulsegenerator 50 to automatically deliver the defibrillation pulse.

While FIG. 3 illustrates certain major components of the defibrillator10, those having ordinary skill in the art will appreciate that thedefibrillator 10 may contain more or fewer components than those shown.The disclosure of a preferred embodiment of the defibrillator 10 doesnot require that all of the general conventional components be shown. Itwill further be appreciated that the invention may be implemented in acardiac monitor having essentially the same components as thedefibrillator 10 shown in FIG. 3, except that the cardiac monitor doesnot have the components necessary for delivering a defibrillation pulse.Furthermore, some or all of the programmed instructions 42, 44 may beimplemented in hardware as an alternative to software instructionsstored in the memory 40.

As noted above, the present invention uses a portion of theimpedance-sensing signal whose frequency range is most likely to revealfluctuations indicating the presence of a cardiac pulse in the patient.The presence of characteristic fluctuations in patient impedanceassociated with a cardiac pulse is used to identify the presence of acardiac pulse in the patient.

FIG. 4 illustrates a pulse detection process 60 conducted in accordancewith the present invention. The pulse detection process 60 uses ananalysis of impedance signal data to determine the presence of a pulsein a patient. Preferably, the impedance signal data selected foranalysis is obtained during time intervals associated with QRS complexesin the patient's ECG.

Beginning at block 70 the pulse detection process 60 captures both ECGand impedance signal data, synchronized in time, for a predeterminedtime interval (e.g., 10 seconds). Preferably, at this time, personsaround the patient are advised to not touch the patient during this timeinterval (e.g., the device could report “analyzing now . . . standclear”). Alternatively, the ECG and impedance capturing step maycontinue until the first or a specified number of QRS complexes in theECG have been identified, or in the event of asystole or a low heartrate, a predetermined maximum period of time (e.g., 10 seconds) haspassed.

In block 72, the pulse detection process 60 locates all of the QRScomplexes in the captured ECG signal. Identification of QRS complexescan be done using methods published in the literature and well-known tothose skilled in the art of ECG signal processing. For example see,Watanabe K., et al., “Computer Analysis of the Exercise ECG: a Review,”Prog Cardiovasc Dis 22: 423-446, 1980.

In block 74, for each time that a QRS complex was identified in the ECGsignal, a segment of filtered impedance signal data obtained from thecaptured impedance data is selected. In one embodiment of the invention,the time window of each segment of impedance data is approximately 600milliseconds in length, and commences prior to the end of the identifiedQRS complex. If no QRS complexes were identified in the captured ECGsignal in block 72 (as would happen for example, during asystole), therewill be no segments of impedance data selected in block 74.

In block 76, one or more measurements are made on a segment of impedancesignal data selected in block 74 to identify or calculate a featureindicative of a cardiac pulse. The measurements may include one or moreof the following:

(1) peak-to-peak amplitude of the impedance signal in the segment(measured in milliohms);

(2) peak-peak amplitude of the first derivative of the impedance signalin the segment (measured in milliohms per second);

(3) energy of the impedance signal in the segment (preferably calculatedby squaring and summing each of the impedance data values in thesegment); or

(4) a pattern matching statistic.

As to the latter measurement (i.e., pattern matching), the segment ofimpedance signal data is compared with one or more previously identifiedimpedance signal patterns known to predict the presence of a pulse. Thecomparison produces a pattern match statistic. Generally, in thiscontext, the greater the value of the pattern match statistic, thecloser the patient's impedance signal matches a pattern impedance signalthat predicts the presence of a pulse. Other candidate measurements willbe apparent to those skilled in the art, and may be used instead of, orin addition to, the aforementioned measurements. A measurement resultingfrom the analysis in block 76 constitutes a feature of the impedancesignal data indicative of the presence of a pulse.

In decision block 78, the one or more features from block 76 areevaluated to determine the presence of a cardiac pulse in the patient.The embodiment shown in FIG. 4 compares the one or more features topredetermined thresholds to determine whether or not a pulse isdetected. For example, an impedance peak-to-peak amplitude measurementwould be consistent with the presence of a pulse if it exceeded acertain threshold (e.g., 50 milliohms). Similarly, an impedance energymeasurement would be consistent with a pulse if its magnitude exceeded apredetermined threshold. Likewise, a pattern matching statistic would beconsistent with a pulse if it exceeded a predetermined threshold. If thefeature exceeded the specified threshold, the pulse detection processdetermines that a pulse was detected, as indicated at block 80. If thefeature did not exceed the specified threshold, a pulse was notdetected, as indicated at block 82. If no segments of impedance signaldata were selected in block 74 (i.e., no QRS complexes were located inblock 72 in the captured ECG), the pulse detection process 60 woulddetermine that a pulse was not detected, as indicated at block 82.

The embodiment shown in FIG. 4 uses thresholding in block 78 todetermine whether a pulse was detected. However, those skilled in theart will recognize other forms of classification and determination thatmay suitably be used in the invention. For example, multi-dimensionalclassifiers may be used in decision block 78 to determine whether apulse was detected. For example, separate analyses of the amplitude andenergy in the impedance data segment, may be performed, with theresultant outcome of each analysis constituting a detection statisticthat is provided to a multi-dimensional classifier. The detectionstatistics may be weighted and compared in the classifier to determinean overall conclusion whether a pulse is present in the patient. Inother embodiments, individual calculations of instantaneous andbackground amplitudes and/or energies may be provided as detectionfeatures for evaluation in a multi-dimensional classifier. Pattern matchstatistics may also be evaluated in the multi-dimensional classifier, asmay other candidate measurements of the impedance signal data.Techniques for constructing multi-dimensional classifiers are well-knownin the art. For an expanded description of classifiers suitable for usewith of the invention, see, e.g., R. Duda and P. Hart, PatternClassification and Scene Analysis, published by John Wiley & Sons, NewYork, and incorporated herein by reference.

After determining whether a pulse was detected (block 80) or notdetected (block 82), the pulse detection process 60 determines whetherall of the segments of impedance signal data selected in block 74 haveanalyzed. If not, the analysis and decision process of block 76, 78, 80,and 82 is repeated for a new impedance data segment. This continuesuntil all of the impedance data segments selected in block 74 have beenanalyzed.

It is recognized that the resulting determination (pulse detected or nopulse detected) may not be the same for each impedance data segmentanalyzed. An additional decision step is used to determine the overalloutcome of the pulse detection process 60. As indicated at decisionblock 86, the pulse detection process 60 may evaluate the determinationsfor each impedance data segment and decide that a pulse is present inthe patient if a pulse was detected in a simple majority of theimpedance segments analyzed. Of course, other voting schemes may beused. If, in decision block 86, a majority is found, the pulse detectionprocess concludes that a cardiac pulse is present in the patient, asindicated at block 90. Otherwise, the pulse detection process 60concludes that the patient is pulseless, as indicated at block 88.

Requiring a pulse to be found in more than a simple majority of theimpedance data segments would improve the specificity of the detection,but decrease the sensitivity for detecting a pulse. Conversely,requiring a pulse to be found for just one impedance segment or for lessthan a majority of the impedance segments would improve sensitivity fordetecting a pulse but decrease specificity. If the pulse detectionprocess 60 concludes that a pulse is present in the patient, the process60 may optionally proceed to check the pulse rate of the patient, asillustrated in FIG. 5. Turning to FIG. 5, in block 92, the number of QRScomplexes (located in block 72 in FIG. 4) are counted. Decision block 94subsequently compares the number of QRS complexes to a threshold. In onepreferred embodiment, the threshold is 5, corresponding to a heart rateof approximately 30 bpm. If the number of QRS complexes is at leastequal to the threshold, the pulse detection process 60 proceeds to block96, concluding that the patient has a pulse and an adequate pulse rate.If the number of QRS complexes is less than the threshold, the pulsedetection process 60 proceeds to block 98, concluding that the patienthas a pulse, but also severe bradycardia.

While a preferred embodiment of the invention as shown in FIG. 4includes capturing both ECG and impedance signal data, and selecting thesegments of impedance signal data based on QRS complexes located in theECG, other embodiments of the invention may not capture or use the ECGsignal. In FIG. 6, an alternative pulse detection process 100 begins bycapturing only impedance signal data from the patient, as indicated atblock 102. Depending on the length of the time interval in whichimpedance data is captured, it may be advantageous to select a segmentof the impedance signal data for further analysis, as indicated at block104. In that regard, one suitable selection process includes scanningthe impedance signal data for the maximum peak and selecting a segmentof data that surrounds the detected maximum peak.

For exemplary purposes, the pulse detection process 100 is shownevaluating the selected segment of impedance signal data using a patternmatch analysis. However, those skilled in the art will recognize thatother techniques (e.g., analysis of the amplitude or energy in theimpedance signal data, as discussed above, may be used.) In block 106,the selected impedance data segment is compared with previouslyidentified impedance signal patterns known to predict the presence of apulse. The resulting pattern match statistic is evaluated against athreshold in decision block 108 to determine whether a pulse wasdetected in the patient. If the pattern match statistic exceeded thethreshold, the pulse detection process 100 concludes in block 110 that apulse was detected in the patient. Otherwise, the pulse detectionprocess 100 concludes that the patient is pulseless, as indicated inblock 112. At this point, the pulse detection process is finished.Alternatively, if a pulse was detected in the patient, the pulsedetection process 110 may proceed to evaluate the patient's pulse ratein a manner described in reference to FIG. 5.

As noted above, the transthoracic impedance signal can containfluctuations due to cardiac pulses, respiration, or patient motion. Toassess whether a patient has a pulse, it is desirable to suppressfluctuations in the patient's impedance that are due to causes otherthan cardiac pulses. Fluctuations due to noncardiac causes may containcomponents at frequencies similar to those of impedance fluctuations dueto cardiac pulses. Consequently, bandpass filtering may not alwaysadequately suppress fluctuations due to noncardiac causes.

Signal averaging of the impedance signal can be used to suppressfluctuations that are due to noncardiac causes. Signal averaging makesadvantageous use of the fact that impedance fluctuations due to cardiacpulses are synchronized to QRS complexes in the ECG signal, whereasother impedance fluctuations are asynchronous to QRS complexes. Pulsedetection may be more accurately accomplished using an averagedimpedance signal.

A preferred method for signal averaging of the impedance signal firststores the continuous ECG and transthoracic impedance signals,synchronized in time, for a predetermined time interval (e.g., tenseconds). The locations of the QRS complexes (if any) in the stored ECGsignal are determined. Using true mathematical correlation (or analternative correlation technique such as area of difference), the QRScomplexes are classified into types, where all QRS complexes of the sametype have high correlation with the first occurring QRS complex of thattype. The dominant QRS type is selected as the type containing the mostmembers, with a preference for the narrowest QRS type when a two or moretypes tie for most members. Using the first QRS of the dominant type asa reference complex, the second QRS complex of the same type is shiftedin time until it is best aligned with the reference complex (i.e., itachieves a maximum correlation value). The corresponding impedancesignal is also shifted in time to stay synchronized with thetime-shifted QRS complex. When the second QRS complex is optimallyaligned with the reference complex, the two QRS complexes are averagedtogether. Their corresponding impedance signals, over a time period fromabout the start of the QRS complex to about 600 milliseconds after theend of the QRS complex, are also averaged together. The averaged QRScomplex is then used as a new reference complex and the process ofaveraging both the QRS complexes and the corresponding impedance data isrepeated with the remaining QRS complexes of the dominant type.

Preferably, during the subsequent averaging of the QRS complexes andimpedance segments, the new QRS complex and impedance segment carry aweight of one and the previous averaged QRS complex and impedancesegment carry a weight equal to the number of QRS complexes that havebeen included in the averaged QRS complex. When all of the QRS complexesof the dominant type have been processed as described above, theaveraged impedance segment is evaluated using one or more of thetechniques previously described (e.g., amplitude, energy, patternmatching), or by using another measuring technique known in the art, todetermine whether or not the patient has a pulse.

During severe bradycardia, there will be few QRS complexes in a10-second period and signal averaging of the transthoracic impedancesignal will not be as effective as when the heart rate is higher.However, at very low heart rates, there is unlikely to be enough bloodflow to support life. For that reason, below a certain heart rate (e.g.,30 bpm), the patient may be considered pulseless.

The pulse detection process of the present invention may be used as partof a shock advisory protocol in a defibrillator for determining whetherto recommend defibrillation or other forms of therapy for the patient.FIG. 7 illustrates a pulse detection/defibrillation process 130,preferably for use in an automated external defibrillator (AED) capableof providing a defibrillation pulse if a patient is determined to bepulseless and in VF or VT.

In the pulse detection/defibrillation process 200 in FIG. 7, the AEDinitializes its circuits when it is first turned on, as indicated atblock 132. The defibrillation electrodes of the AED are placed on thepatient. When the AED is ready for operation, the process 130 performsan analysis of the patient, as indicated at block 134, in which the AEDobtains selected parameters such as impedance signal data and ECG datafrom the patient. During the analysis performed in block 134, the AEDpreferably reports “Analyzing now . . . stand clear” to the operator ofthe AED.

Using the information obtained in the patient analysis, the process 130determines in decision block 136 whether the patient is experiencingventricular fibrillation (VF). If VP is present in the patient, theprocess 130 proceeds to block 142 where the AED prepares to deliver adefibrillation pulse to the patient. In that regard, an energy storagedevice within the AED, such as a capacitor, is charged. At the sametime, the AED reports “Shock advised” to the operator of the AED.

Once the energy storage device is charged, the process 130 proceeds toblock 144 where the AED is ready to deliver the defibrillation pulse.The operator of the AED is advised “Stand clear . . . push to shock.”When the operator of the AED initiates delivery of the defibrillationpulse, the process 130 delivers the defibrillation shock to the patient,as indicated in block 146.

The AED preferably records in memory that it delivered a defibrillationpulse to the patient. If the present pulse delivery is the first orsecond defibrillation shock delivered to the patient, the process 130may return to block 134 where the patient undergoes another analysis. Onthe other hand, if the pulse delivery was the third defibrillation pulseto be delivered to the patient, the process 130 may proceed to block 140where the AED advises the operator to commence providing CPR therapy tothe patient, e.g., by using the message “Start CPR.” The “No shockadvised” prompt shown in block 140 is suppressed in this instance. TheAED may continue to prompt for CPR for a predetermined time period,after which the patient may again be analyzed, as indicated in block134.

Returning to decision block 136, if VF is not detected in the patient,the process 130 proceeds to decision block 138 and determines whether acardiac pulse is present in the patient. The pulse detection performedin block 138 may be one of the pulse detection processes 60 or 100described above.

If, at decision block 138, a pulse is detected in the patient, theprocess 130 proceeds to block 139 and reports “Pulse detected . . .start rescue breathing” to the operator. The process 130 may also report“Return of spontaneous circulation” if a pulse is detected in thepatient any time after the delivery of a defibrillation pulse in block146. In any event, after a predetermined time period for rescuebreathing has completed, the process 130 preferably returns to block 134to repeat an analysis of the patient.

If a cardiac pulse is not detected at decision block 138, the process130 determines whether the patient is experiencing ventriculartachycardia (VT) with a heart rate of greater than a certain threshold,e.g., 100 beats per minute (bpm), as indicated at decision block 141.Other thresholds such as 120, 150, or 180 bpm, for example, may be used.If the determination at decision block 141 is negative, the process 130proceeds to block 140 and advises the operator to provide CPR therapy.Again, at this point, the AED reports “No shock advised . . . start CPR”to the operator. The prompt to provide CPR is provided for a definedperiod of time. When the period of time for CPR is finished, the process130 preferably returns to block 134 and performs another analysis of thepatient. If the determination at decision block 141 is positive (i.e.,the patient is experiencing VT with a heart rate greater than thethreshold), the process 130 performs the shock sequence shown at blocks142, 144, 146 to deliver a defibrillation pulse.

Variations and additions to the process 130 within the scope of theinvention are recognized by those having ordinary skill indefibrillation and cardiac therapy. FIG. 8, for example, illustrates analternative pulse detection/defibrillation process 150 for use in anAED. As with the process 130 in FIG. 7, the AED begins by initializingits circuits at block 152. At block 154, the AED performs an analysis ofthe patient in a manner similar to that described with respect to block134 in FIG. 7. After completing the analysis of the patient, the process150 proceeds to decision block 156 to determine whether a pulse ispresent in the patient. The pulse detection performed in block 156 maybe, for example, any one of the pulse detection processes 60 or 100discussed above.

If a pulse is detected in the patient, the process 150 may enter amonitoring mode at block 158 in which the patient's pulse is monitored.The pulse monitoring performed at block 158 may use any one or acombination of the pulse detection processes described herein.Preferably, the process 150 is configured to proceed from block 158 toblock 154 after expiration of the predetermined monitoring time period.If the pulse monitoring at block 158 determines that at any time a pulseis no longer detected, the process 150 returns to block 154 to performanother analysis of the patient. The process 150 also preferably reportsthe change in patient condition to the operator.

If, at decision block 156, a pulse is not detected in the patient, theprocess 150 proceeds to decision block 160 where it determines whetherthe patient has a shockable cardiac rhythm (e.g., VF or VT). Asreferenced earlier, U.S. Pat. No. 4,610,254, incorporated herein byreference, describes a suitable method for differentiating shockablefrom non-shockable cardiac rhythms.

If a shockable cardiac rhythm, such as VF or VT, is detected, theprocess 150 proceeds to a shock delivery sequence at blocks 162, 164,and 166, which may operate in a manner similar to that described withrespect to blocks 142, 144, and 146 in FIG. 7. If the pulse delivery wasthe third defibrillation shock delivered to the patient, the process 150may proceed to block 168 and prompt the delivery of CPR, as discussedwith block 140 in FIG. 7.

If VF or VT is not detected at decision block 160, the process 150checks for asystole, as indicated at block 167. One suitable process fordetecting asystole is described in U.S. Pat. No. 6,304,773, assigned tothe assignee of the present invention and incorporated herein byreference. If asystole is detected at block 167, the process 150proceeds to prompt the delivery of CPR, as indicated at block 168. Ifasystole is not detected, the process 150 determines that the patient isexperiencing pulseless electrical activity (PEA), as indicated at block169. PEA is generally defined by the presence of QRS complexes in apatient and the lack of a detectable pulse, combined with no detectionof VT or VF. As described above, detection of PEA in block 168 isachieved by ruling out the presence of a pulse (block 156), detecting noVF or VT (block 160), and detecting no asystole (block 167).Alternatively, if the ECG signal is monitored for QRS complexes (e.g.,as shown at block 70 in FIG. 4), the process 150 may conclude thepatient is in a state of PEA if it repeatedly observes QRS complexeswithout detection of a cardiac pulse associated therewith. If a PEAcondition is detected, the process 150 proceeds to block 170 and promptsthe operator to deliver PEA-specific therapy to the patient. Onesuitable method of treating PEA is described in U.S. Pat. No. 6,298,267,incorporated by reference herein. The process 150 may prompt othertherapies as well, provided they are designed for a PEA condition. Aftera PEA-specific therapy has been delivered to the patient, possibly for apredetermined period of time, the process 150 returns to block 154 torepeat the analysis of the patient.

FIG. 9 illustrates yet another pulse detection/defibrillation process200 that may be used in an AED. At block 202, after the AED has beenturned on, the AED initializes its circuits. The defibrillationelectrodes are also placed on the patient. The AED is then ready toanalyze the patient, as indicated at block 204. This analysis may beperformed in a manner similar to that described with respect to block134 in FIG. 7.

If at any point the AED determines that the defibrillation electrodesare not connected to the AED, the process 200 jumps to block 206 wherethe AED instructs the operator to “Connect electrodes.” When the AEDsenses that the electrodes are connected, the process 200 returns to theanalysis in block 204. Likewise, if the AED finds itself in any otherstate where the electrodes are not connected, as represented by block208, the process 200 jumps to block 206 where it instructs the operatorto connect the electrodes.

Furthermore, during the analysis performed in block 204, if the AEDdetects motion on the part of the patient, the process 200 proceeds toblock 210 where the AED reports to the operator of the AED “Motiondetected . . . stop motion.” If the patient is moved during the analysisprocess 204, the data obtained during the analysis is more likely to beaffected by noise and other signal contaminants. Motion of the patientmay be detected in the impedance signal data collected by the presentinvention. A suitable method for detecting motion of the patient isdescribed in U.S. Pat. No. 4,610,254, referenced earlier andincorporated by reference herein. The AED evaluates the impedancemeasured between the defibrillation electrodes placed on the patient. Asnoted earlier, noise and signal components resulting from patient motioncause fluctuations in the impedance signal, generally in a frequencyrange of 1-3 Hz. If the measured impedance fluctuates outside of apredetermined range, the AED determines that the patient is moving orbeing moved and directs the process 200 to proceed to block 210. Whenthe motion ceases, the process 200 returns to the analysis in block 204.

The process 200 next proceeds to decision block 212 where it determineswhether a pulse is detected in the patient. Again, the pulse detectionprocesses performed in decision block 212 may be, for example, one ofthe pulse detection processes 60 or 100 described above.

If a pulse is not detected in the patient, the process 200 proceeds todecision block 214 where it determines whether the patient has ashockable cardiac rhythm (e.g., VF or VT or a non-shockable cardiacrhythm (such as asystole and bradycardia). As referenced earlier, onesuitable method for differentiating shockable from non-shockable cardiacrhythms is disclosed in U.S. Pat. No. 4,610,254, incorporated herein byreference. If the patient's cardiac rhythm is determined to be shockable(e.g., VF or VT is found), the process 200 proceeds to blocks 216, 218,and 220 to deliver a shock to the patient. The shock delivery may beperformed as described earlier with respect to block 142, 144, 146 inFIG. 7.

If the pulse delivery was the third defibrillation pulse to be deliveredto the patient, the process 200 proceeds to block 222 where the AEDadvises the operator to commence providing CPR therapy to the patient.The CPR prompt may continue for a defined period of time, at which theprocess 200 returns to block 204 and performs another analysis of thepatient.

If, at decision block 214, the patient's cardiac rhythm is determinednot shockable, the process 200 preferably proceeds to block 222 andadvises the operator to provide CPR therapy, as discussed above.

Returning to decision block 212, if a pulse is detected in the patient,the process 200 proceeds to decision block 224 where it determineswhether the patient is breathing. In that regard, the AED may again usethe impedance signal for determining whether a patient is breathing. Asnoted earlier, fluctuations in impedance of the patient below 1 Hz arelargely indicative of a change in volume of the patient's lungs. Thebreathing detection at block 224 (and at blocks 226 and 228, discussedbelow) may monitor the impedance signal for characteristic changes thatindicate patient breathing, e.g., as described in Hoffmans et al.,“Respiratory Monitoring With a New Impedance Plethysmograph,” Anesthesia41: 1139-42, 1986, and incorporated by reference herein. Detection ofbreathing may employ a process similar to that described above fordetection of a pulse (i.e., evaluating impedance amplitude, energy, orpattern), though a different bandpass filter would be used to isolatethe frequency components that more closely demonstrate patientbreathing. If automatic means for detecting breathing in the patient arenot available, the AED may ask the operator of the AED to inputinformation (e.g., by pressing a button) to indicate whether the patientis breathing.

If, at decision block 224, the process 200 determines that the patientis not breathing, the process 200 proceeds to a block 226 where theoperator of the AED is advised to commence rescue breathing. In thatregard, the AED reports to the operator “Pulse detected . . . startrescue breathing.” The AED also continues to monitor the patient'scardiac pulse and returns to block 204 if a cardiac pulse is no longerdetected. If, at any point during the provision of rescue breathing, theAED detects that the patient is breathing on his own, the process 200proceeds to block 228 where the AED monitors the patient for a continuedpresence of breathing and a cardiac pulse.

Returning to decision block 224, if the process 200 determines that thepatient is breathing, the process 200 proceeds to block 228 where theAED monitors the pulse and breathing of the patient. In that regard, theAED reports “Pulse and breathing detected . . . monitoring patient.” If,at any time during the monitoring of the patient the process 200determines that the patient is not breathing, the process 200 proceedsto block 226 where the operator of the AED is advised to commence rescuebreathing. If a cardiac pulse is no longer detected in the patient, theprocess 200 proceeds from block 228 to block 204 to commence a newanalysis of the patient.

Lastly, as noted in FIG. 9, during the rescue breathing procedure inblock 226 or the monitoring procedure performed in block 228, the AEDmay assess whether CPR is being administered to the patient. If the AEDfinds that CPR is being performed, the AED may prompt the operator tocease providing CPR. If, during the CPR period of block 222, the AEDdetermines that CPR is not being administered to the patient, the AEDmay remind the operator to provide CPR therapy to the patient. Onemethod for determining whether CPR is being administered is to monitorpatient impedance to observe patterns of impedance fluctuation in thepatient that are indicative of CPR. During CPR, repetitive chestcompression typically causes repetitive fluctuations in the impedancesignal.

FIG. 10 illustrates yet another application in which the pulse detectionprocess of the present invention may be used. The process described inFIG. 10 pertains to auto-capture detection in cardiac pacing.

Specifically, the auto-capture detection process 250 begins at block 252in which pacing therapy for the patient is initiated. A counter N,described below, is set to equal 0. At block 254, a pacing pulse isdelivered to the patient. Thereafter, filtered impedance signal data isobtained from the patient, as indicated at block 256. The impedance datais used in block 258 to detect the presence of a cardiac pulse in thepatient. The pulse detection process used in block 258 may be one of thepulse detection processes 60 or 100, discussed above.

The sequence of delivering a pacing pulse and determining the presenceof a cardiac pulse in blocks 254, 256, 258 is repeated a predeterminednumber of times. With respect to FIG. 10, for example, the sequence isrepeated five times. At block 260, the counter N is evaluated, and ifnot yet equal to 5, the counter is incremented by 1 (block 262),following which the process 250 returns to deliver another pacing pulseto the patient.

If, at decision block 260, the counter N equals 5, the process 250determines at decision block 264 whether a cardiac pulse occurredconsistently after each pacing pulse. The process 250 requires that someportion or all of the pacing pulses result in a detectable cardiac pulsebefore pronouncing that capture has been achieved. If the presence of acardiac pulse is determined consistently follow the pacing pulses, theprocess 250 determines that capture has been achieved, as in indicatedat block 266. Otherwise, the current of the pacing pulses is increasedby a predetermined amount, e.g., 10 milliamperes, as indicated at block268. At block 270, the counter N is set back to equal 0 and the process250 returns to the pacing capture detection sequence beginning at block254. In this manner, the pacing current is increased until capture hasbeen achieved.

In FIG. 10, the presence of a pulse is used to determine whether thepacing stimulus has been captured by the ventricles. Detection of QRScomplexes in the patient's ECG may also be used to identify pacingcapture. In that regard, the patient's ECG would be monitored alongwith, or in place of, the impedance data collection in block 256. A QRScomplex will occur immediately following the pacing stimulus if capturehas been achieved. If QRS complexes are not observed, the current of thepacing pulses may be increased, as discussed above, until capture hasbeen achieved.

FIG. 11 illustrates still another application in which the pulsedetection process of the present invention may be used. The process 280described in FIG. 11 is particularly suited for use in a manualdefibrillator or patient monitor. Beginning at block 282, the process280 monitors the patient's ECG for QRS complexes. At block 284, theprocess 280 also obtains filtered impedance signal data from thepatient. The process 280 uses the ECG and impedance signals in decisionblock 286 to determine the presence of a pulse. The pulse detectionimplemented in block 286 may be one of the pulse detection processes 60or 100.

If a pulse is detected, the process 280 determines whether adefibrillation pulse has been provided to the patient and if so, reportsthe return of spontaneous circulation to the operator, as indicated atblock 298. The process 280 then returns to block 282 to repeat the pulsedetection analysis. If a pulse is not detected, the process 280evaluates the ECG signal to determine whether the patient isexperiencing ventricular fibrillation or ventricular tachycardia with aheart rate greater than 100 bpm. If so, then the process identifies thepatient's condition and sounds a VT/VF alarm, as indicated at block 290.If not, the process 280 then proceeds to block 292 to check for anasystole condition.

Detection of asystole may be accomplished as noted earlier and describedin greater detail in U.S. Pat. No. 6,304,773, incorporated herein byreference. If asystole is detected, the process 280 identifies thepatient's condition and sounds an asystole alarm, as indicated at block294. Otherwise, the patient is experiencing PEA and the patient'scondition is so identified, with the sound of a PEA alarm, as indicatedat block 296. In this manner, the operator of the manual defibrillatoror monitor is kept advised of the patient's condition.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1-7. (canceled)
 8. A method of determining the presence of a cardiacpulse in a patient, comprising: (a) obtaining data derived from animpedance-sensing signal that has been communicated from the patient;(b) analyzing the obtained impedance signal data for a featureindicative of the presence of a cardiac pulse, wherein analyzing theimpedance signal data includes comparing the impedance signal data to apreviously-identified impedance signal data pattern known to predict thepresence of a cardiac pulse; and (c) determining whether a cardiac pulseis present in the patient based on the feature in the impedance signaldata.
 9. The method of claim 8, wherein the comparison produces apattern match statistic that is the feature indicative of the presenceof a cardiac pulse, the method further comprising comparing the featureto a predetermined pattern match threshold to determine whether acardiac pulse occurred in the patient.
 10. The method of claim 8,further comprising analyzing the impedance signal data for two or morefeatures indicative of the presence of a cardiac pulse, wherein one ofthe features is determined from the comparison of the impedance signaldata with a previously-identified impedance signal data pattern andwherein one of the other features is determined from an evaluation of anamplitude of the impedance signal data or energy in the impedance signaldata.
 11. The method of claim 8, further comprising prompting deliveryof chest compressions or cardiopulmonary resuscitation to the patient ifa cardiac pulse is determined not present in the patient.
 12. The methodof claim 8, further comprising: (a) obtaining electrocardiogram (ECG)data from the patient; and (b) determining the presence of a QRS complexin the ECG data, wherein analyzing the obtained impedance signal datafor a feature indicative of the presence of a cardiac pulse furtherincludes determining whether a QRS complex occurred in the ECG data. 13.The method of claim 12, further comprising locating a QRS complex in theECG data and selecting impedance signal data for the pattern matchcomparison based on the located QRS complex.
 14. The method of claim 8,wherein analyzing the obtained impedance signal data further includesdetermining cardiac output in terms of a stroke volume or rate ofoutput, wherein the determined cardiac output is used as an additionalfeature indicative of the presence of a cardiac pulse, and wherein thecardiac output feature is compared to a predetermined output thresholdto determine whether a cardiac pulse is present in the patient. 15-23.(canceled)
 24. A medical device, comprising: (a) electrodes adapted forplacement on a patient to sense an impedance-sensing signal that hasbeen communicated from the patient; (b) a conversion circuit incommunication with the electrodes for converting the sensedimpedance-sensing signal into digital impedance signal data; and (c) aprocessing unit in communication with the conversion circuit forprocessing the impedance signal data, wherein the processing unit isconfigured to (i) analyze the impedance signal data for a featureindicative of the presence of a cardiac pulse by comparing the impedancesignal date to a previously-identified impedance signal data patternknown to predict the presence of a cardiac pulse, and (ii) determinewhether a cardiac pulse is present in the patient based on the featurein the impedance signal data.
 25. The medical device of claim 24,wherein the comparison produces a pattern match statistic that is thefeature indicative of the presence of a cardiac pulse, the processingunit being further configured to compare the feature to a predeterminedpattern match threshold to determine whether a cardiac pulse is presentin the patent.
 26. The medical device of claim 24, wherein theconversion circuit and the processing unit are implemented in adefibrillator.
 27. The medical device of claim 26, wherein thedefibrillator is an automated external defibrillator.
 28. The medicaldevice of claim 24, wherein the processing unit is further configured toanalyze the impedance signal data by determining a cardiac output interms of a stroke volume or rate of output and use the determinedcardiac output as an additional feature indicative of the presence of acardiac pulse, the processing unit being further configured to comparethe additional feature to a predetermined cardiac output threshold todetermine whether a cardiac pulse is present in the patient.
 29. Themedical device of claim 24, further comprising a display incommunication with the processing unit, wherein the processing unit isfurther configured to prompt a message on the display reporting whethera cardiac pulse is present in the patient.
 30. The medical device ofclaim 24, further comprising a display in communication with theprocessing unit, wherein the processing unit is further configured toprovide a graph on the display showing a representation of the impedancesignal data.
 31. The medical device of claim 24, further comprisingelectrodes adapted to sense electrocardiogram (ECG) signals in thepatient, wherein the conversion circuit further converts the ECG signalreceived from the patient into digital ECG data, and wherein theprocessing unit is further configured to analyze the ECG data, determinethe presence of a QRS complex in the ECG data, and select impedancesignal data corresponding in time with the QRS complex for the analysisof the impedance signal data.
 32. The medical device of claim 24,wherein the processing unit is configured to analyze the impedancesignal data for two or more features indicative of the presence ofcardiac pulse, wherein one of the features is determined from thecomparison of the impedance signal data with a previously-identifiedimpedance signal data pattern and wherein one of the other features isdetermined from an evaluation of an amplitude of the impedance signaldata or an energy impedance signal data. 33-38. (canceled)